Apparatus for generating a pressure wave in an elongated body operatively connected to a drop hammer

ABSTRACT

When driving piles into the ground the drop hammer should act upon the head of the pile through the intermediation of an elastically deformable cap. The resistance of the ground will vary from one occasion to the other and will furthermore often vary during the driving of an individual pile. This makes it desirable to change the elastic properties of the cap, which to that end is designed to enclose a body of gas and a body of liquid, means being provided for adjusting the volume and the pressure of the gas by governing the flow of liquid from one chamber to another within the cap.

United States Patent 1191 Ludvigson Mar. 19, 1974 APPARATUS FOR GENERATING A 3.417.828 12/1968 Duyster et a1. 175/19 PRESSURE WAVE IN AN ELONGATED 32221532 ghassagne ..l 1;

, sen BODY OPERATIVELY CONNECTED To A 3.646.598 2/1972 Chelminski l 61/515 DROP HAMMER 3.714.789 2/1973 Chelminski 61/535 7 Inventor: Berger Ludvigson, sadra Vagen 38 3.721.095 3/1973 Chelminski 61/535 412 54 Goteborg, Sweden Primary Examiner-Gerald M. Forlenza [22] Fned' 1972 Assistant Examiner-R. Johnson [21] Appl. No.: 298,494 Attorney. Agent. or Firm-Holman & Stern [30] Foreign Application Priority Data 57 ABSTRACT Oct. 18,1971 Sweden ....131 34 71 when driving piles into the ground the drop hammer should act upon the head of the pile through the inter- [52] Cl 32 5 2 22 4 mediation of an elastically deformable cap. The resis- [51] Int Cl 825d 9/00 Ezlb 11/O2 lfi 27/08 tance of the ground will vary from one occasion to the [58] Fie'ld 'g 267/l37 l 182 173/1 other and will furthermore often vary during the driv- "'"i'I 6i/53 ing of an individual pile. This makes it desirable to change the elastic properties of the cap, which to that end is designed to enclose a body of gas and a body of [56] References cued liquid, means being provided for adjusting the volume UNITED STATES PATENTS and the pressure of the gas by governing the flow of 675.319 5/1901 Boyd 173/131 X liquid from one chamber to another within the cap. 2.184.745 12/1939 Kinneman... 267/137 X 2.723.532 11/1955 Smith 267/137 X 9 Claims, 7 Drawing Figures PATENTEDMARWIQM I 3.797585 SHEEI 1 BF. 6

FIG]

PATENTEU MAR I 9 I974 SHEH 2 [IF 6 APPARATUS FOR GENERATING A PRESSURE WAVE IN AN ELONGATED BODY OPERATIVELY CONNECTED TO A DROP HAMMER BACKGROUND OF THE INVENTION The present invention refers to a method and a means for generating a pressure wave in an elongated body, one end which is acted upon by a drop hammer 1 through the intermediation of an impact cap designed to be elastically deformed in the direction of the impact, to bring about a driving of piles, sheet pilings, tubes or the like into the ground.

Piles are usually driven by means of repeated strokes of short and heavy drop hammer acting upon the top of the pile, called the pile head. If the drop hammer is permitted to contact the pile head directly the stresses therein during each moment of the impact will be proportional to the speed of the hammer. As the latter is rapidly braked by the counteraction of the pile, the pressure wave brought about by the impact will show a short and steep peak, the curve thereafter decreasing exponentially. As the resistance to entrance (and thus the carrying capasity, or the breaking load of the pile) presupposes a certain amount of resistance in the pile to be overcome, while simultaneously the peak stress must not endanger the strength of the pile, it is evident that the exponentially falling stress curve is unsatisfactory, and that the portion of the pressure wave, useful for the driving of the pile, is very small indeed. Such an arrangement, thus, has a very low efficiency.

By fitting an impact cap between the pile and the drop hammer the shape of the impact wave may be changed in an advantageous manner. Usually a block of soft wood is introduced between the hammer and the pile. This type of cap will however never be ideal, as the properties of the wooden block will be altered during the piling operation and makes the calculation of the result of the hammering, and of the carrying capacity of the driven pile difficult.

SUMMARY OF THE INVENTION The aim of the present invention is to propose a method, which on the one hand makes it possible to drive piles in a more safe and efficient manner than hitherto, and on the other hand provides a better base for calculating the carrying capacity of the pile after driving. In order to reach this aim the impact cap is provided with means for adjusting the impact force in accordance with the occasional working conditions. The movements of the head of the pile, or the changes of stresses in said head, during and after the impact are measured by ocular inspection or by an instrument. and the impact force is adjusted according to these readings. The impact force is selected in such a manner that the head of the pile will be at rest and the stresses therein will be constant, when the pressure wave has passed.

The means according to the invention is characterized in that the cap is designed as a cylinder in which a piston is movable due to the action of the drop hammer against the action of a body of gas, the volume and pressure of which is determined by readings of the movements of the head and/or the changes of the stresses therein.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will below be described with reference to the accompanying drawings, which refer to theoretical background and shows some preferred embodiments.

FIG. 1 shows the main component of one embodiment of the invention, as utilized for pile driving,

FIG. 2 is a diagram showing the cooperation between 0 drop hammer, impact cap and pile,

FIG. 3 schematically shows one embodiment of an impact cap,

FIG. 4 shows a second embodiment of an impact cap,

FIG. 5 shows a modified embodiment of the upper part of the impact cap according to FIG. 4, and

FIG. 5 shows a further embodiment of the impact cap.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the theoratically ideal case the force transfering the inertia of the 13 134/71 hammer to a pressure Wave within the pile shall be constant during the time of the impact. The pressure wave within the pile will then obtain the rectangular shape most suitable for pile driving. Under such conditions the head of the pile will move with constant speed in the direction of impact, as long as the impact remains.

The dropping moving of the hammer will simultaneously be braked. The braking force is constant, as is the retardation. The desirable constant speed at retardation. pile head prohibits a direct contact between drop hammer and pile. The inertia must be transferred to the pile by means of a body, which permits deformation in the direction of impact, and which in the ideal case, above referred to, has such properties that the force causing the deformation also is constant.

The aims of the invention are explained by the following analysis, which describes the theoretical qualifications for an efficient utilization of the inertia of the drop hammer during the work to overcome the resistance of the ground to the entrance of the pile. The method according to the invention is illustrated by the principal arrangement according to FIG. 1, in which guides and operating means are removed.

The drop hammer 1, the weight of which is H, drops against the elastic member 2 (here called the impact pile cap) which is compressed by a constant, or almost constant, force P. This force P will also act upon the pile 3 and causes a compression (or a pressure wave) which moves from the head of the pile towards the point thereof with the velocity of sound, c, within the material of the pile, and the potential energy of which is utilized for overcoming the resistance of the ground to the entrance of the pile.

The hammer 1 is during the movement towards the cap accelerated by the force H, said acceleration being the same as the acceleration due to gravity, g. At the moment of impact the hammer 1 has the velocity v,,. During the impact the hammer l is first retarded and then accelerated in a direction upwards. The force which acts upon the hammer 1 is composed partly of the force F from the cap 2, directed upwards, and partly of the weight H of the hammer l, which is directed' downwards. The acceleration of the hammer 1 during the impact is a P- H/H For a normal case duringpile driving the value of 1-7 is small when compared to P, and the acceleration a can simply be written as a E (P/H)g.

The velocity of the hammer l at the point after the moment of impact is v v, a t. When the velocity of the hammer 1 is zero it has transferred all its inertia to the cap 2. This means that the time of impact t v /a is a condition for optimum utilization of the inertia of the hammer.

The part w of the total energy, which at the point of time, t, is transferred to the cap 2, is w t/t (2t/t,) and the distance over which the hammer 1 moves after the moment of impact is obtained by S v,, t a t /2.

During the impact the head of the pile is acted upon by the force P from the compression of the cap. The velocity of the head will then be v Pc/EA. The velocity is constant and will, beside by P, be determined by the velocity of sound, c, within the pile the cross sectional area A of the pile, and the modulus of elasticity E of the material in the pile. The movement of the head after the moment of impact S =v t.

The compression of the cap corresponds to the difference between the distance of movement of the hammet and of the pile, respectively, i.e. S S S (v v,,) t a U /Z). This means that the compression is O at point T= 2/a (v v,,), and it is evident that v, must be bigger than v,, in order to bring about a compression. So the desired force P is obtained.

At the point T= 2/a (v, v,,) the compression of the cap has returned, to the position before the impact. This means that no energy transferred to the cap by way of the impact remains therein.

If this point coincides with the movement when the inertia of the drop hammer 1 is 0, i.e. when the velocity of the hammer is zero all the inertia possessed by the hammer 1 at the moment of impact has been transferred to the pile. The condition for this optimum transfer of energy between hammer and pile is that v 2 v,,, i.e. the velocity of the hammer l at the moment of impact must be twice as big as the velocity of the head of the pile, brought about by the action of force P from the cap.

The inertia of the hammer is now completely transferred to a potential energy in the pile and appears as a pressure wave with the pressure force P and the length Tc. The pressure wave is so long that the Weight of the part of the pile, which is covered by the wave, is equal to twice the weight of the hammer, i.e. TcAy=2H (y=the weight ofa unit of volume of the material in the pile). This further means that the relation above referred to is invalid if the weight of the hammer 1 is bigger than that of the pile 3. If the hammer is too heavy the .velocity of the pile head will be disturbed during the later part of the impact period by waves reflected from the point of the pile.

The cooperation between hammer, cap and pile is illustrated grafically in FIG. 2. The hammer moves from the moment of impact (t=0) along a parabolic curve A-B-C. Derivatives of this parabola determine the velocity of the hammer at each point of time. The inclination of line A-A, which is a tangent to the parabola at point A, shows the velocity of the hammer at the moment of impact v,,. If line A-B represents the movement of the pile head as a function of time, the difference be' tween the parabolic curve A-B and the straight line A-B represents the compression of the cap, and the inclination ofline A-B denotes the velocity v,, of the pile head.

It is evident that if v,, v /2, then line A-B will intersect the parabola at vertex (point B), where the velocity of the hammer is zero, (and the compression of the cap also is zero).

If v,, v /2, which is illustrated by line A-D, it is evident that the compression of the cap is zero only when the hammer has got an upwardly directed movement with a velocity which is defined by the derivative of the parabola at point D. This velocity corresponds to inertia which is loast, if it cannot be transferred to work for lifting the hammer towards a new stroke.

lf v,,/2 v,, v it is evident that the compression of the cap has returned to zero while the hammer-still moves downwards. The pressure wave in the pile will then be determinated by an exponentially falling pressure curve, which brings about a lower strain in the pile.

If v,, v no compression will occur in the cap and the pile driving will occur as if no cap was at hand.

Concerning the cooperation of the pile and the ground when the potential energy of the pressure is utilized for driving the pile into the ground reference is made to H C Fischer On Longitudinal Impact, Uppsala 1960. In volume III Fischer refers to a case, where the resitance of the ground is supposed to consist of a frictional force F, which is concentrated to the point of the pile. Fischer introduces the idea of relative friction, f, which is defined as f F/P.,, where R, is the force appearing at the point of the pile when the pressure wave is reflected against a solid substrate. With the symbols used above P 2P.

Fischer further shows that the part w" of the energy of the pressure wave, which is utilized for the driving, depends upon f according to the equation w" 4f lf).

lff l then w" will be negative. This means that the frictional force F is too big, and no entrance of the pile is obtained. The energy of the pressure wave is reflected and appears at an upwardly directed pressure wave. For each value of the resistance to entrance, F, there thus is a minimum value of force P, which must be exceeded in order to make the pile sink during the stroke.

is l f /z then O w 1. Part ofthe energy will then be used for forcing the pile into the ground., while the remaining part of the energy (l-w") is reflected upwards along the pile.

If the length of the pressure wave Tc 2L, where L is the length of the pile, the reflected wave will reduce the impact pressure P during the latter part of the period of impact T. As above explained Tc 2L when H ALy.

For the value FVz is w"=l which means that all impact energy transferred to the pile has been utilized for the driving work.

If /2 f 0 the reflected pressure of the wave at the point of the pile causes a tensile stress in the pile. The resulting pressure wave is again reflected from the head of the pile and a new pressure wave runs towards the point of the pile. This secondary pressure wave may, if the intencity thereof is sufficient, assist in the sinking of the pile. If Tc 2L the secondary pressure wave will assist the primary pressure wave, the impact period of which has not yet termnated.

Such a hard hammering may bring about a good efficiency. The pulling stresses appearing in the pile will, however, on certain occasions, be detrimental and they will with concrete piles be acceptable only if the piles are strengthened by longitudinal reinforcements to such an extent that a detrimental cracking is prevented.

This short resume of Fisher's paper gives at hand that the full impact energy will be utilized for the driving work only if force F in the cap is equal to the resistance to entrance. Neither a bigger force, nor a smaller force will provide the same advantageous shall utilisation of the energy. At the head of the pile this advantageous utilisation of the energy may be observed thereby that the head of the pile will remain at rest after the impact, and will not be disturbed by any reflected pressure wave, neither in the direction of the stroke nor in the opposite direction. This paper thus confirms the earlier postulation that in the theoretically ideal case the force P in the cap shall be constant during the time of impact.

In his paper Fischer later on relates a case, where the resistance to entrance of the ground is considered as frictional forces distributed along the length of the pile, and cases where the resistance to entrance is combined with an elastic resistance. All these cases may be referred back to the first mentioned report.

To sum up it may be verified that for each combination of ground conditions, resistance to entrance and weight of pile and hammer, respectively, there are definite values of the drop height h of the hammer, and of the impact force P from the cap, which combined with each other provide a maximum utilisation of the energy supplied by the impact for driving the pile.

As the conditions of the ground, as well as the resistance to entrance, will vary during the driving of each pile means must be provided at the cap for altering the pressure force P in such a manner that this will be suited to the occasional conditions. An amendment of P will also mean adjustment of the drop height h of the hammer in order to make the energy transfer as high as possible. I

In a earlier method for driving piles an elastic body was introduced between the hammer and the pile, or alternatively a device was used, which beforehand was biased by a pressure sufficient to guarantee that the force transferred to the piles during the impact is at least as big as the resistance of the ground and also less than the highest force accepted by the pile without damage.

From the statements above it is evident that a cap, which before the driving operation is biased to have these properties, will not provide a guarantee for a high efficiency during all driving operations. In order to obtain this desired high efficiency the cap shall, beside the above mentioned properties, be designed in such a manner that the biasing of the cap is suited to the ground conditions in such a manner that the force transferred to the pile during the impact is equal to the resistance and also so the drop height of the hammer (or the velocity thereof at the moment of impact) is suited to the biasing of the cap, whereby the velocity at the moment of impact v, is equal to twice the velocity v,, of the pile head.

The method for driving piles according to one embodiment of the invention presupposes an impact cap, which permits a deformation in the direction of impact and which transfer an impact force of predetermined magnitude to the pile. The suitability of the magnitude of the impact force utilized is derived from the movement of the pile head after the impact, or from the stresses in the pile when the pressure wave has passed once. These readings, which may be made ocularly or by means of an instrument, provide a basis for the selection of thyimpact force with respect to the occasional working conditions, and the impact cap is provided with means for bringing about this adjustment. The drop height of the hammer is observed ocularly or by means of an instrument to ascertain that the hammer is at rest when the deformation of the cap has returned to the starting point.

An impact cap according to this embodiment of the invention will be explained below with reference to FIG. 3.

One end of a cylinder 4 is closed by a piston 5, the outwardly movement of which is determined by a shoulder 6 at the cylinder 4. The opposite end of the cylinder is closed by a piston 7, the position of which is determined by a volume of liquid in a chamber 10 defined in the cylinder 4 by a shoulder 8, the piston 7 and a rod 9 connected thereto. The chamber which is defined within cylinder 4 by pistons 5 and 7 is partly filled with a gas 11 under high pressure and partly by liquid 12. The volume of liquid 12 communicates with the volume of liquid in chamber 10 by way of an adjustable valve 13 and two passages provided with nonreturn valves 14, which are designed in such a manner that they for a certain position of valve 13 permits liquid to pass in the direction from 12 to 10 and for another position of valve 13 permits liquid to pass from 10 to 12. Valve 13 can be moved to a fully closed position, in which it prevents communication between 10 and 12. Sealing means 15 are provided between cylinder 4 and pistons 5, 7 and rod 9. i

The pile driving is brought about by permitting the hammer 1 to drop from height 11,, above piston 5, and it will meet the latter with a velocity v 2 3 h,,. The piston 5 will be forced into cylinder 4 the interior of which is partly filled with gas 11 under high pressure. When piston 5 by hammer l is forced from its outwardly position shown on the drawing, the gas 11 will act on the one hand against the hammer 1, which is retarded, and on the other hand by way of the liquid 12 and piston 7 9 against the pile 3. A pressure wave will be generated in the pile 3, which brings about a downwardly movement at the head of the pile with the velocity v,,, the magnitude of which is proportional to the pressure of the gas 11. The value of the drop height h and the pressure of the gas 11 is selected in such a manner that v,, E 2v For each value of the pressure of the gas 1 l a certain value of the drop height h may be calculated, which provides the best utilization of the impact energy during the pile driving operation. In practice this optimum energy transfer may be checked by observing the movements of the hammer after the impact. If too big a drop height h is utilized then v 2 v and the hammer 1 will leave the piston 5 after the impact and perform an upwardly directed movement, the energy consumed thereby being lost to the pile driving. The hammer 1 will in other words bounce back from piston 5. The energy transfer will be w 4 v /v (l v /v If the height of the fly back is denoted by h, the energy transfer may also be explained as w l h lh The values of h, and h, may be observed during the driving operation, and h a be selected in such a manner that 11, will be small and approaches zero. The checking hereof may occur with or without an instrument.

The pressure wave generated in pile 3 will pass with the velocity of sound towards the point of the pile. When the pressure in the gas 11 is selected in the proper way with respect to the conditions of the ground all potential energy of the pressure wave will be utilized for the driving operation and the pile head will remain at rest when the impact period has passed.

If on the other hand the pressure of gas 11 is too high the pile head at point t= 2L/c after the moment of impact will be pulled downwards by a pressure wave deflected from the point of the pile. This may be observed during the driving operation thereby that the hammer 1 will be suspended free during a moment before it once again falls towards the pile.

If the pressure in the gas 11 is too low the reflected wave will appear as a pressure wave which, when it reaches the pile head, will knock the cap 2 away in the direction upwards.

A proper pressure of gas 11 is thus a condition to ensure a high efficiency. The pressure is adjusted by means of valve 13. If the gas pressure at 11 is too high valve 13 is adjusted in such a manner that liquid may pass from to 12, which is possible between the impact periods when the pressure at 10 is higher than the pressure at 12. The piston 7 will then be displaced in such a manner that the volume 11 of the gas will be bigger and the pressure reduced.

If the pressure of the gas 11 is too high to suit the ground conditions valve 13 is adjusted in such a manner that liquid may pass from 12 to 10, which is possible during the time of the impact. The pressure at 10 is then lower that at 12.

An adjustment of the gas pressure 11 may usually be required several times during the driving of a single pile, depending upon the changes in the resistance of the ground and will generally increase with the depth. The means for adjusting valve 13 is not shown in FIG. 3, but can be designed to operate with mechanical or hydraulical transfer and be adjusted manually or by means of a motor.

The hammer will be lifted and released for each stroke, for instance by means of hoist or by an automatically working device, for instance utilizing the exhaust gas pressure of a internal combustion engine, acting as working cylinder.

Valve 13 may be substituted by a reversible pump 23, which transfers fluid from 10 to 12, or in the opposite direction. Such an arrangement is illustrated in FIG. 4, which shows a modified embodiment of the invention, in which the impact cap described in connection with FIG. 3 has been developed in such a manner that the lifting of the hammer may be obtained by way of the expanding gas 11.

FIG. 2 graphically shows how the hammer 1 from the moment ofimpact, in the time axis denoted by point A, moves along the parabola A-B-C under the influence of force P from the cap. At point C the hammer 1 has the same velocity v,,, as at the moment of impact, but oppositely directed. If the time of the impact is interrupted at point C the hammer 1 will be returned to the starting point of the dropping movement, and the cycle would be repeated.

During the time of impact t, represented by distance A-C, the head of the pile has been displaced the length v z. In order to bring about an automatic operation of the pile driving the gas 11 in cap 2 before each stroke should be compressed to a value corresponding to the The length of the impact time is 2v,,/a,,, i.e. twice as deformation v t. This biasing shall be released during the stroke and provides the hammer l with an addition of energy Iiy t, which is required for automatic big as the pressure time obtainable with the arrangement according to FIG. 3. This means that the efficiency of the pile driving is doubled.

This embodiment of the invention will be explained more closely in relation to FIG. 4.

One end of cylinder 4 is closed by a piston, arranged concentrically with the cylinder 4 and designed as a cylinder 16, one end of which is closed and the other end of which is open towards the space enclosed in cylinder 4. Cylinder 16 is provided with sealing means at the end 6 of cylinder 4, as well as at an annular valve member 17, which may slide along and is sealed against the internal wall of cylinder 4. Valve member 17 cooperates with a shoulder 18 at cylinder 16. The opposite end of cylinder 4 is designed in principally the same manner as described in connection with FIG. 3. The valve means 13 of FIG. 3 is however substituted by a reversible pump 23 having driving means 24. Gas 1 1 of high pressure is enclosed in cylinder 16. The remaining spaces are filled with liquid. The pressure of gas 11 forces cylinder 16 towards an upper position, in which member 17 abuts against shoulder 6 and seals against shoulder 18.

A further pump 19 driven by a further motor 20 transfers fluid from a chamber 12 to the annular chamber 21, which is defined within cylinders 4 and 16 and by valve member 17. Cylinder 16 will then be forced into cylinder 4 by the pressure inchamber 21, during which the gas 11 will be further compressed.

When a suitable additional energy has been transferred to gas 11 the hammer 1 will fall against cylinder 16, whereby the gas 11 will be further compressed, while simultaneously the valve will open at 18 and valve member 17 will return to its upper position by means of a spring 22, while liquid passes between the external face of cylinder 16 and valve member 17, from 21 to 12. The length of the stroke during the compression ofgas 11 during the action of hammer 1 will in this manner be stroke then the length of the stroke during the expansion of the gas. The hammer 1 will be thrown back towards its starting point for a new dropping movement, and is retained in this position by suitable means. Before the hammer 1 once again is released to move against cylinder 16 for a further impact pump 19 has brought about the desired high pressure of gas 11, which means that piston 5 will be forced downwards in the cylinder. The cycle is repeated as long as the pump 19 works and the gas 11 recoils hammer l to such a height that the velocity v at the moment of impact is bigger than the velocity of the pile head v,, during the impact. This is a condition to make valve at 18 to open.

An embodiment which works independently of the velocity of the drop hammer 11 is shown in FIG. 5. The valve at 12 (FIG. 4) is here substituted by a valve, governed by a spigot 25, which extends downwards through the end portion of cylinder 16 turned towards the hammer. This spigot 25 is connected to an annulus 26, which may be brought to cover a number of passages 27 in the wall of cylinder 16. In the position shown in the drawing liquid is transferred from chamber 12 to chamber 21 in the same manner as above described. When hammer l forces the spigot 25 inwards the connection between 12 and 21 by way of passages 27 is opened, whereby the gas 11 may expand to act upon the hammer and the pile. If the product of gas pressure 11 multiplied by the crossectional area of spigot 25 is smaller than the weight of the hammer it is evident that valve 27 will open independent of the velocity of the hammer at impact. This embodiment will however not imply any limitation for the case where v, v,,.

A further modification of the automatically driven hammer is also shown in FIG. 5. In the part of cylinder 16 turned towards chamber 12 a valve body 28 is fitted, which normally permits passage of liquid between the spaces within cylinders 16 and 4 (12). When cylinder 16 is subjected to an impact, during which is presupposed that v v cylinder 16 will be forced into cylinder 4. Valve body 28 will due to its own inertia and to the difference in pressure between chambers 11 and 12 seal between the end portion of cylinder 16. The result hereof will be an occasional increased impact, which later, during expansion of the gas 11, is transferred into an impact force, which is determined by the pressure of the gas 1 1. The embodiment is suitable with ground conditions providing a higher initial resistance for the pile than later on during its movement.

In FIG. 4 it has been presupposed that the liquid is contained in different chambers within the cylinder, and is permitted to flow between the said chambers in governed quantities. The alternative embodiment shown in FIG. 6 includes a reservoir, within the impact cap, separated from the cylinder proper ll, 12. This cap is provided with a single piston 5, which is designed as a hollow cylinder 16, the enlarged end 31 of which seals against the internal wall of cylinder 4. The liquid is transferred from the reservoir to the cylinder, and vice versa, by means ofa reversible pump 23, driven by a motor 24. Valve member 32 is axially displaceable in the chamber, defined between piston and the upper part of the cylinder, said member being designed in one position to seal against the upper face of the enlarged portion 31 of piston 5. The pump 19, driven by a motor 20, transfers liquid from reservoir 30 to chamber 34, whereby the gas 11 is provided with additional energy, which will be released by the impact of the hammer. The cap operates in the same manner as described in connection with FIG. 4, with the difference that liquid is transferred during the impact from 34 to 30, passing between the outside of cylinder 16 and member 32 and through the one-way valve 33.

FIG. 7 shows an alternative embodiment of FIG. 5. As in FIG. 6 a reservoir is built into the cap, separated from the chambers 11, 12. When the hammer forces the spigot inwards valve body 26 is displaced and the connection from chamber 34 to reservoir 30 by way of passages 36, 37 and 33 is opened.

In further embodiments the reservoir for the liquid may be separated from the cap and will communicate with the latter by way of liquid transferring conduits.

The drop hammer l and the cap 2 have been described as separate elements. Alternatively the drop hammer may be combined with, or connected to either the piston 5 or the cylinder 4, whereby the free part of the cap is designed to act upon the pile during the impact.

The additional energy transferred to gas 11 to provide an automatic operation may be obtained by designing chamber 34 of FIGS. 6 and 7 as connected to the working cylinder of an internal combustion engine, of a hot air engine.

Common for these automatically operating embodiments is that the lifting of the hammer is brought about by means of the expanding gas 11, and that the stroke of cylinder 16 compared to that of cylinder 4 is shorter during compression than during expansion of the gas 1 l.

In the embodiments described gas at high pressure has been used the medium taking up the deformation during the impact period. Instead of compressed gas any other elastic medium or elastic body, or a combination of elastic materials of different kinds may be utilized.

The frequency of the drop hammer I may be increased by an elastic device 29 (FIG. 1) which increases the acceleration of the hammer during the drop movement. Braces for the forces from the device 29 may be arranged in suitable manner in the pile driving rig or the guides thereof, which are not shown on the drawing.

I claim:

1. A device for generating a pressure wave in an elongated body, one end of which, the head is acted upon by a drop hammer through the intermediation of a cap permitting an elastic deformation in the direction of impact, said cap comprising a cylinder and a piston biased by an elastic body enclosed in the cylinder and operable therein by the action of the drop hammer, the improvement of means defining at least two chambers within the cap a body of liquid within said chambers and means for selectively governing the flow of liquid between the chambers for governing the volume and the pressure of the elastic body.

2. The device as claimed in claim 1 in which two pistons are operable towards and away from each other in the cylinder, a first one acted upon by the hammer and the other acting upon the elongated body, said pistons defining between themselves a first chamber enclosing the elastic body and a selected portion of the liquid.

3. The device according to claim 2 in which the second piston is provided with a first passage for communication with said first chamber, and with two second passages for communication with a second chamber and with the first passage, the flow governing means being located intermediate all three passages to govern flow through them, each of the second passages being provided with a non-return valve, one being devised to prevent flow to the first chamber, and the other to permit flow thereto.

4. The device according to claim 2 in which the flow governing means includes a reversible pump for transferring liquid from the first chamber to the second chamber, and vice versa.

5. The device according to claim 2 in which the first piston is designed as a hollow cylinder open towards the second chamber and adapted to receive a gaseous elastic body.

6. The device according to claim 2, in which the first piston is provided with an enlarged portion defining a third chamber within the cylinder, the flow govering means comprising a reversible pump for transferring liquid between the first and the second chambers and a further pump being provided for transferring liquid from the first chamber to the third chamber.

hammer is fitted within the hollow first piston for opening a passage between the second and the third chamber.

9. The device according to claim 8; in which an automatically operating valve member is fitted in the hollow, first piston to define the portion of the second chamber containing the elastic body from the main portion of the chamber containing the liquid. 

1. A device for generating a pressure wave in an elongated body, one end of which, the ''''head'''' is acted upon by a drop hammer through the intermediation of a cap permitting an elastic deformation in the direction of impact, said cap comprising a cylinder and a piston biased by an elastic body enclosed in the cylinder and operable therein by the action of the drop hammer, the improvement of means defining at least two chambers within the cap a body of liquid within said chambers and means for selectively governing the flow of liquid between the chambers for governing the volume and the pressure of the elastic body.
 2. The device as claimed in claim 1 in which two pistons are operable towards and away from each other in the cylinder, a first one acted upon by the hammer and the other acting upon the elongated body, said pistons defining between themselves a first chamber enclosing the elastic body and a selected portion of the liquid.
 3. The device according to claim 2 in which the second piston is provided with a first passage for communication with said first chamber, and with two second passages for communication with a second chamber and with the first passage, the flow governing means being located intermediate all three passages to govern flow through them, each of the second passages being provided with a non-return valve, one being devised to prevent flow to the first chamber, and the other to permit flow thereto.
 4. The device according to claim 2 in which the flow governing means includes a reversible pump for transferring liquid from the first chamber to the second chamber, and vice versa.
 5. The device according to claim 2 in which the first piston is designed as a hollow cylinder open towarDs the second chamber and adapted to receive a gaseous elastic body.
 6. The device according to claim 2, in which the first piston is provided with an enlarged portion defining a third chamber within the cylinder, the flow govering means comprising a reversible pump for transferring liquid between the first and the second chambers and a further pump being provided for transferring liquid from the first chamber to the third chamber.
 7. The device according to claim 6 in which an annular valve member operable by the liquid pressure is fitted in the third chamber, cooperating with the enlarged portion of the first piston for govering flow of liquid between the second and the third chamber past said enlarged portion of the piston
 8. The device according to claim 6, in which an annular valve member mechanically operable by the drop hammer is fitted within the hollow first piston for opening a passage between the second and the third chamber.
 9. The device according to claim 8, in which an automatically operating valve member is fitted in the hollow, first piston to define the portion of the second chamber containing the elastic body from the main portion of the chamber containing the liquid. 